During recent years, alternative fuels have attracted considerable interest. This is particularly true of fuels based on renewable sources as, for instance, wood, straw, and other types of biomass, including also a variety of wood- and biomass waste. Also peat, even though not renewable, may be mentioned in this connection.
The use of such materials for fuel purposes can often be facilitated if they are available in particulate form. A typical example is wood flour used as replacement for oil in boilers etc. The average particle size of such material will, as a rule, depend on the type of the use it is intended for. In general, a far going reduction of the particle size is desirable. When burning particulate fuels in boilers one thus finds that smaller particles give a shorter flame which is easier to control. The importance of a sufficiently small particle size becomes especially obvious when using particulate fuels in gas turbines or diesel engines. In the first case, the burning process must be completed during the short passage of the particle through the turbine, whereas in the latter case the fuel cannot behave in the intended manner if the particle size has not been reduced sufficiently before the injection in the combustion chamber of the diesel engine.
It is known that grinding of biomass of various provenience to fuels with sufficiently small particle size--one could mention as an example an average size of 10 .mu.m--is a highly energy consuming process. It is also known that a chemical pretreatment of the biomass intended for fuel purposes has been proposed in order to facilitate the size reduction. In the first place, a pretreatment using acidic substances has been proposed, preferably various acids normally used as dilute water solutions. Such a treatment breaks the glycosidic linkages of the cellulose, thus reducing the length of the molecular chains. This, in turn, brings about a marked embrittlement of the material treated in such manner. Effects of this type can be observed both with pure cellulose and with various types of lignocellulosic materials, as wood, unbleached cellulose fibre etc. Examples of embodiments of such embrittlement can, for instance, be found in the U.S. Pat. No. 3,961,913 which describes the treatment of the organic fraction of municipal solid waste with acid solutions at elevated temperatures, in order to convert this fraction to powder. Another example is the embrittlement of wood chips using dilute sulphuric acid in order to make possible the comminution of the wood substance into fine particles before it is converted in a reactor into a liquid fuel (Schalegger, L.L. et al.: Sol. Energy Syst. Inst., Tech. Rep. 1979, TP-33-285, 3rd Ann. Biomass Energy Syst. Conf., 1979, Proc., 119-22). One can here also mention the steaming of wood at temperatures in excess of 150.degree. C. as described in the Swedish Pat. No. 84851. During such treatment a number or organic acids are liberated through chemical reactions, which, in turn, results in the embrittlement discussed here.